1. Field of the Invention
The present invention generally relates to a racket stringer and, more particularly, is concerned with a string tensioning force controlling apparatus for a racket stringer.
2. Description of the Prior Art
A traditional racket stringer has a rock arm onto which weights are manually added. By means of different weights, the rock arm is turned to create different torque for driving a string pulling head to pull and tension a racket string. The racket string is tensioned by different poundage.
Even if, in accordance with the above arrangement, a heavy weight is used to tension the string, the operation by being a substantially manual one, still often results in great error. Moreover, the weights gradationally vary from one another such that the poundage of the tensioned string cannot be micro-adjusted.
One prior art racket stringer employs a motor for driving a gear set to create torque for driving the string pulling head and tensioning the string. The motor, gear set and string pulling head form a string pulling mechanism pivotally disposed on a base seat of the racket stringer. When the string pulling head is turned to pull and tension the string to a certain poundage, the reaction force of the string pulling head will drive a rock member of the string pulling mechanism to swing. This racket stringer also employs a controlling unit which has a sensor (such as a strain meter, a variable resistor or a limit switch) for controlling the motor. The rock member once driven to swing will contact with the sensor. The contact force applied by the swinging rock member with the sensor is detected as the change of the resistant force of the sensor and in response thereto an analog signal is generated and sent to a decoder of the controlling unit for calculating the tensioning force exerted onto the string by the string pulling mechanism.
The sensor which generates the analog signal, such as the strain meter, is likely to produce error due to the change of environmental factors, such as temperature. The decoder of the controlling unit which receives the analog signal converts it into a digital signal for facilitating the calculation of the tensioning force. The decoding procedure carried out by the decoder reproduces the error of the analog signal in the digital signal. Furthermore, generally it is the one end of the rock member distal from the pivoted section that contacts with the sensor. The length of the rock member serves as a force arm for magnifying the collision force. Such design can easily produce error of the value detected by the sensor. This leads to error in the output analog signal generated by the sensor. Thus, the detection measurement provided by the output analog signal contains considerable error so that the pulling tension exerted onto the racket string by the prior art string pulling mechanism cannot be accurately controlled. Another disadvantage to this prior art racket stringer is that the wiring from the sensor moves with the string pulling head while tensioning the string. This can result in premature wear to the sensor wires and eventually they will need to be replaced.
Another prior art string tensioning device known as the “Molstring” under its commercial name, is comprised of a base frame, a gripping drum for gripping the string rotatably mounted to the base frame, a hand crack arrangement for applying tension to string rotatably mounted to the base frame and fixed to the gripping drum, a lever arm fixed to the hand crank arrangement on one end and attached to the base frame on the other end with a biasing spring, where the lever arm is also rotatably mounted to the base frame. An absolute digital encoding device is employed where the sensed member of the encoding device is fixed to the lever arm, and the sensor of the encoding device is fixed to the base frame. As tension is manually applied to the string by turning the hand crank arrangement, the lever rotates relative to the base frame and moves the sensed member of the absolute digital encoder relative to the sensor of the absolute digital encoder to send a signal to a central processing unit and display the tension being applied to the string on an LED display. The tension in the string is controlled manually by observing the LED display and is very difficult to control to achieve the desired tension in the string. Furthermore, the absolute digital encoder has discrete indicators positioned on it such that each signal generated at each indicator is uniquely associated to a discrete and specific tension level being applied to the string and therefore must be perfectly mounted relative to the sensor and perfectly matched to the load verses deflection (spring rate) characteristics of the biasing spring in order to display the correct tension. Due to variations in the spring rate of different springs, the biasing spring is rarely matched perfectly to the discrete indicators on the sensor member, resulting in inaccurate tension display. This makes controlling the tension applied to the string and achieving the desired tension in the string difficult, if not impossible.
Consequently, there is a need for an innovation that will overcome the aforementioned problems without introducing any new problems in place thereof.